1. Field of the Invention
This invention relates to a focusing screen adapted to be mounted in the optical viewing path of a camera.
2. Description of the Prior Art
Cameras of the single lens reflux type and also of the range finder type generally include a focus indicating system utilizing an image splitting bi-prism. This focus indicating system usually comprises a focusing screen having a focus indicating part disposed in the center of the real image plane of the range finder and comprising image splitting prisms.
The focus indicating system of such split-image type, as compared with other systems, has a good focusing accuracy, but suffers from a phenomenon that the focus indicating part dark-changes for an interchangeable lens having a large F-number, namely, one low in brightness.
As the aperture ratio of the lens is decreased, the image splitting bi-prism has a larger dark area. Accordingly, the focusing becomes more difficult with increasing F-number of the lens.
This focus indicating accuracy and the darkness of the focus indicating part have a relation contrary to each other (inversely proportional to each other) for the vertical angle of the image splitting prism. This contrary relation is attributable to the fact that, among the light beams from the exit pupil of the lens, only the light beam having a particular angle of incidence determined by the vertical angle of the image splitting prism passes through the range finder and reaches the eye of the observer and contributes to the focus indicating. That is, if the vertical angle of the splitting prism is increased, the light rays from the marginal portion of the exit pupil of the phototaking lens can be directed to the eye-piece of the view finder and thus, the focus indicating accuracy is enhanced. However, a splitting prism having a great vertical angle has its focus indicating part dark-changeable even for an interchangeable lens which is a little dark, namely, which has a large F-number, and thus, the focus indicating becomes impossible.
Conversely, if the vertical angle of the prism is made small, the focus indicating accuracy is reduced but the focus indicating is possible even for an interchangeable lens which has a large F-number, namely, which is considerably dark.
To overcome such phenomenon, the vertical angle of the image splitting prism usually is not rendered to so great a value but is minimized to an angle in the vicinity of 8.degree. so that, even if the focus indicating accuracy is sacrificed to some extent, the focus indicating becomes possible even for a dark interchangeable lens having an F-number of the order of 5.6.
However, for interchangeable lenses of a smaller maximum aperture ratio, for example, f/8 and larger, the aforementioned problem of dark prism is again presented.